It is the conventional practice to apply heat treatment to a hot-rolled steel sheet for the purpose of improving strength, toughness and other properties of the hot-rolled steel sheet. In most cases, such a heat treatment is applied to the hot-rolled steel sheet allowed to spontaneously cool after the completion of hot rolling. However, since such a heat treatment is very low in efficiency, apparatus have recently been developed, which cool a steel sheet immediately after the completion of hot rolling before the temperature of the steel sheet lowers to below a prescribed value, thereby improving strength, toughness and other properties thereof.
As one of the apparatus as mentioned above for cooling a steel sheet immediately after the completion of hot rolling, the following apparatus has been proposed, as disclosed in Japanese Patent Publication No. 11,247/78 dated Apr. 20, 1978, which comprises:
a plurality of upper and lower support/guide rollers each arranged symmetrically relative to the plane of a steel sheet laid horizontally; and a covering including a substantially flat wall arranged between two adjacent rollers on each side of the steel sheet and a wall surrounding said two adjacent rollers, said covering being closed at the both ends and the both side edges thereof, and a plurality of cooling water supply pipes and a plurality of cooling water discharge pipes being alternately connected to said wall surrounding said two adjacent rollers, thereby cooling the steel sheet through contact of the upper and the lower surfaces of the steel sheet with cooling water in said covering.
A steel sheet immediately after the completion of hot rolling can be cooled by the above-mentioned apparatus which has however the following disadvantages:
1. Since cooling water flows in the covering which provides only a limited space, it is difficult to control the cooling rate. PA1 2. Necessity to cool the steel sheet by cooling water while moving the steel sheet makes it impossible to start cooling the entire steel sheet at a time. Therefore, the cooling start temperature cannot be the same between the leading end and the trailing end in the longitudinal direction of the steel sheet. PA1 1. to easily control the cooling rate, since cooling water from the cooling nozzle units is not subjected to any constraint; and, PA1 2. to cool at a time the entire steel sheet placed on the table. PA1 1. Cooling water from the upper cooling nozzle units, which is ejected onto the upper surface of the steel sheet flows down from the both side edges of the steel sheet. When considering the steel sheet in the width direction thereof, therefore, the side edge portions are cooled more strongly than the center portion.
With these disadvantages in view, another apparatus has been developed, as disclosed in Japanese Utility Model Publication No. 28,194/81 dated July 4, 1981, which permits starting of cooling of a steel sheet at a time under easy control of the cooling rate, and cools the steel sheet by cooling water ejected from cooling nozzles, which comprises:
a table comprising a plurality of rollers for placing thereon substantially horizontally a steel sheet immediately after the completion of hot rolling; and a plurality of upper cooling nozzle units and a plurality of lower cooling nozzle units respectively arranged, at prescribed intervals in the longitudinal direction of said steel sheet placed on said table, above and below said steel sheet, each of said cooling nozzle units having substantially the same length as the width of said steel sheet, each of said cooling nozzle units being arranged in parallel with the width direction of said steel sheet, and said plurality of upper cooling nozzle units and said plurality of lower cooling nozzle units being adapted to eject cooling water respectively onto the upper and the lower surfaces of said steel sheet.
With the above-mentioned apparatus equipped with the cooling nozzle units, it is possible:
According to the above-mentioned apparatus equipped with the cooling nozzle units, it is possible to cool a steel sheet immediately after the completion of hot rolling, which has an average surface temperature of 600.degree. to 900.degree. C., for example, to a temperature up to 550.degree. C. at a cooling rate of, for example, 3.degree. to 15.degree. C./sec.
In general, however, the temperature distribution in the width direction of a steel sheet immediately after the completion of hot rolling is not uniform. More particularly, as shown in FIG. 1(a), the temperature of a steel sheet immediately after the completion of hot rolling is lower at the side edge portions in the width direction than at the center portion thereof. Therefore, when cooling a steel sheet immediately after the completion of hot rolling by an apparatus equipped with the cooling nozzle units as mentioned above, the difference in temperature between the side edge portions and the center portion in the width direction of the steel sheet immediately after the completion of cooling would further be enlarged as shown in FIG. 1(b) for the following reasons:
2. Because of the complicated heat conduction mechanism shown by water cooling in the high temperature region, the side edge portions are cooled more strongly than the center portion in the width direction of the steel sheet.
In the steel sheet thus cooled, therefore, there is a serious deviation in mechanical properties such as tensile strength in the width direction and the entire steel sheet demonstrates an insufficient flatness as a whole. An example of an average surface hardness distribution in the width direction of a steel sheet thus cooled and then allowed to spontaneously cool is shown in FIG. 1(c).